The invention relates to polyester-containing coil coating compositions and a process for preparing them.
Coil coatings are applied to coiled sheet metal stock, such as steel or aluminum, in an economical, high-speed process. Coil coating is a continuous operation, with the end of one coil typically being attached to the beginning of the following coil to be coated. Important properties for coil coated metals include weatherability, water resistance, chemical resistance, scratch resistance, gloss, hardness, flexibility, and resistance to surface delamination or cracking upon bending of the substrate. The latter property is important since the coated metal typically is subjected to a forming step during the preparation of end use articles.
Many coil coating compositions are known to those skilled in the art. However, formulation of coil coating compositions involves a trade-off wherein improved flexibility is only attained at the expense of hardness, and vice versa. U.S. Pat. No. 6,897,265 B2 shows at least one example of a coil coating composition having a Pencil Hardness of at least 2H and a T-bend test value of 0T. However, this result is only obtained by using a blend of a branched polyester and a linear polyester. It would be desirable to have a simplified coil coating composition that could be used to prepare coatings having an excellent combination of hardness and flexibility.